Nice suggestion, but:
1. Certain plates are already invulnerable to some weapons. They have "minimal armour value", which multiplied by volume gives the minimal damage threshold. So the mechanic is already in game, just the numbers need balancing.
Not sure if old damage formulas still apply, but that would give the pistol ~18 projectile energy. So oninum plate of volume above 72Kv (72x96 plate) should be totally invulnerable to pistol rounds.
EDIT: just tested in practice, with assault rifle: 144x144 oninum requires 1 assault rifle mag to start loosing voxels. 2 mags for 192x192, 432x432 have eaten ~40 mags before I got bored - totally invulnerable to AR.
2. Maximum single plate size is limited by how much endo can grab. Which is around 40 tons. So your suggested plates shouldn't exceed volume of ~1.5 stack, otherwise it would be unpickable if made of oninum. It's already a problem with few existing plates, let's not make it worse.
3. IMO no special magic rules should be made for those plates. That would only invalidate normal plates. I'd rather have the entire damage system improved to be more voxel-based and less part-hp based, so any size of plate is viable.
So IMO damage should be first reduced by [material*effective thickness*% of remaining voxels] to filter out weapons too weak to even scratch it.
Then should be applied to part HP pool [material*part volume] to prevent instant voxel damage from spray weapons that would affect performance. It shouldn't be too noticeable while using proper weapons.
Then should slowly chew through voxels, with tougher materials requiring more projectile energy per removed voxel. That should be the main stopping point.